The use of Raschel knitted nettings for wrapping objects such as pallet loads is known in the Industry. Raschel knitted nettings usually include longitudinal ribbons or threads, known as franze or warp yarns, and of lateral ribbons or threads, known as schuss or fill yarns, which form a triangular structure between each pair of longitudinal ribbons. Such a Raschel knitted netting is described in U.S. Pat. No. 5,104,714.
Due to the triangular geometrical structure, such knitted nettings exhibit lateral shrinkage upon longitudinal elongation (i.e., there is narrowing of the net when it is stretched lengthwise). This problem with Raschel knitted nettings of triangular structure and the solution to this problem are disclosed in U.S. Pat. No. 6,521,551, which is incorporated by reference in its entirety.
These knitted nettings, which are intended, inter alia, for wrapping loads on pallets, usually have a characteristic elasticity and a predetermined degree of elongation capacity. The knitted nettings have to stretch according to the elongation percentage suited to the type of netting being used. There is direct connection between the required tension and the netting's elongation percentage, as a function of the elongation characteristics of the material from which the net is manufactured.
The knitted netting elongates as a function of the tension applied to the netting, irrespective of whether this tension is created upon initiation of wrapping by the wrapping machinery, or, typically at a higher percentage, due to forces created by the object being wrapped.
During the use of pallet nettings for wrapping loads on pallets, the knitted nettings are commercially elongated between 15% and 170% at present according to the characteristics of the netting and the settings of the wrapping machinery. An elongation of x %, where x is a real number, shall mean herein that the netting elongated by x % has a length of (100+x) % as compared to its original length.
The operator of the wrapping machinery endeavors to set the degree of elongation to a target value taking into consideration various factors such as the desired tension, the type of goods wrapped, the elongation capability of the knitted netting etc, all the above in order to optimize the wrapping and the utilization of the netting's characteristic. Specific percentage of elongation and tension is required in order to achieve good wrapping. If the elongation and tension is lower than that required, the load will not be properly secured and the operator will not utilize, and benefit from, the entire elongation capability of the netting. On the other hand, if the elongation and tension percentage exceeds the desired one, the netting can narrow, and this may result in the products wrapped (or their packaging) becoming damaged, e.g. by crushing of corners and cutting of products, or the netting can break or lose its strength leading to insufficient wrapping.
However, it is difficult for the operator of the wrapping machinery to determine the elongation percentage of the netting which will bring about the desired wrapping result. For example, in order to calculate the percentage of elongation, prior to commencing the wrapping process, the operator may measure the length of a predetermined portion of the netting (e.g. ten triangular bases) between two parallel Franze ribbons. Thereafter, the operator may measure the length of said predetermined portion of the netting at the end of the wrapping cycle while the netting is on the machinery, and deduce the percentage of elongation. This procedure, however, is tedious and time-consuming, and may interrupt the wrapping process, increasing the processing time.
If the elongation of the netting is not correctly determined, an undesired elongation may result, causing, as explained above, excess pressure on the wrapped products, damage to the wrapped products, loss of process time and loss of money.
Consequently, there is a need for an improved netting and for a method for determining or measuring elongation of a netting, overcoming the above problems.